a decade of organic polymeric photovoltaic research

7/25/2019 A Decade of Organic Polymeric Photovoltaic Research

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25th Anniversary Article: A Decade of Organic/PolymericPhotovoltaic Research

Letian Dou,Jingbi You,Ziruo Hong,Zheng Xu,Gang Li,* Robert A. Street,*andYang Yang*

L. Dou, Dr. J. You, Dr. Z. Hong, Dr. Z. Xu, Dr. G. Li,

Prof. Y. YangDepartment of Materials Science and EngineeringUniversity of California, Los AngelesLos Angeles, CA 90095, USAE-mail: [email protected]; [email protected]. Dou, Prof. Y. YangCalifornia Nano Systems InstituteUniversity of California, Los AngelesLos Angeles, CA 90095, USADr. R. A. StreetPalo Alto Research CenterPalo Alto, CA 94304, USAE-mail: [email protected]

DOI: 10.1002/adma.201302563

1. Introduction

Photovoltaic (PV) technology, which generates electricitydirectly from sunlight, is a promising solution to the energycrisis. Intensive research is searching for high efficiency solarcells with low-cost fabrication. Currently, various inorganicmaterials (for example, silicon (Si), III-V group semicon-ductors, CdTe, CIGS) based PV devices are the dominatingtechnologies in the market.[1] However, partially due to thehigh production cost and related environmental issues, con-ventional PV technology hasnt successfully replaced grid-electricity. At the current stage, electricity generated fromthe PV accounts less than 0.1% of total US energy genera-tion, and similarly, worldwide.[2] In recent years, there hasbeen growing interest in organic-based PV (OPV) tech-nology. Organic semiconductors show great promise owing

to their synthetic variability, low-tem-perature processing, and the possibilityof producing lightweight, flexible, easilymanufactured, and inexpensive solarcells.[38] Organic electronic devices,such as organic light-emitting diodes(OLEDs), organic field effect transistors(OFETs), and organic memory devices,have attracted considerable attentionas well.[914] Among them, OLEDs havebeen successfully commercialized.

Unlike inorganic semiconductors, inwhich atoms are covalent-bonded in three-dimentions, organic conjugated moleculesor polymers are individual molecularsemiconductors.[1517] Within a molecule,the pzatomic orbitals of the carbon atom(also for the nitrogen, oxygen, sulfuratoms, etc.) overlap to form conjuga-tion molecular orbitals and the electrons

are delocalized within the conjugated backbone. Inter-molec-ular interactions are formed through Van der Waals and aro-matic interactions, which play important roles in the solidstate.[16,17]The energy difference between the highest occupiedmolecular orbital (HOMO) and lowest unoccupied molecular

orbital (LUMO) levels determines the bandgap of the mate-rial. The organic semiconductors can be p-doped or n-dopedby removing electrons from the HOMO level or adding elec-trons to the LUMO level, respectively. Typically, materials withrelatively high HOMO level can be easily p-doped and transportholes and are thus called p-type (or electron donor), while mate-rials with relatively low LUMO level can be easily n-doped andtransport electrons and are called n-type (or electron acceptor).

Electrically, there are two major distinctions between organicand inorganic semiconductors. First, the charge carriers movefreely in the conjugated backbone, but the inter-molecularcharge transport is much more difficult for organic materials.Hopping from one molecule to the adjacent molecule limitsmacroscopic charge transport. As a result, the charge mobility

of organic materials is significantly lower than the mobilityof inorganic materials, such as crystalline Si (103 versus103cm2V1s1).[11,12,18]However, the high absorption coefficientof organic chromophores allows organic semiconductors to cap-ture most of the photons (within the absorption range) using avery thin layer (100200 nm) and avoid severe charge recom-bination. Second, tightly bound Frenkel excitons (electronholepairs) in organic materials are observed, resulting from theirlow dielectric constant (r 24). The binding energy of theFrenkel exciton is in the range of 0.31 eV, whereas the bindingenergy of the Wannier exciton (free electron hole pairs at roomtemperature) in the inorganic semiconductors is comparable to

Organic photovoltaic (OPV) technology has been developed and improved

from a fancy concept with less than 1% power conversion efficiency (PCE)

to over 10% PCE, particularly through the efforts in the last decade. The

significant progress is the result of multidisciplinary research ranging from

chemistry, material science, physics, and engineering. These efforts include

the design and synthesis of novel compounds, understanding and control-

ling the film morphology, elucidating the device mechanisms, developing

new device architectures, and improving large-scale manufacture. All of these

achievements catalyzed the rapid growth of the OPV technology. This review

article takes a retrospective look at the research and development of OPV,and focuses on recent advances of solution-processed materials and devices

during the last decade, particular the polymer version of the materials and

devices. The work in this field is exciting and OPV technology is a promising

candidate for future thin film solar cells.

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thermal energy at room temperature 26 meV.[19,20]Therefore,to separate the tightly bound exciton effectively, an organic het-ero-junction that consists of two distinct materials (n- and p-type, respectively) with sufficient energy band offset is needed.Due to these characteristics of organic semiconductors, verydifferent device architectures and processing techniques for the

organic solar cell are employed to achieve high power conver-sion efficiency (PCE).Three important parameters determine the PCE of a solar

cell: the open-circuit voltage (VOC), short-circuit current den-sity (JSC), and fill factor (FF). The PCE is equal to the productof these three parameters divided by the input power. [21] Thesimplified working principle of an OPV device is comprised offour steps: (1) photon absorption and exciton generation, (2)exciton diffusion, (3) charge separation, and (4) charge collec-tion.[38] The external quantum efficiency (EQE) is defined asthe numerical ratio of the collected photo-generated chargesand the incident photons at certain wavelength, and it is deter-mined by the efficiency of the above four steps. The J SC of asolar cell is also equal to the integral of the product of cellresponsivity (or EQE as a function of wavelength) and incidentsolar spectral irradiance.[21] It is worth pointing out that otherimportant phenomena can influence the PCE, such as ultrafastcharge transfer, hot excitons, etc.

The fundamental knowledge of organic semiconductors andphotovoltaic devices was obtained through a long journey ofresearch. The first successful OPV device was reported as earlyas 1986 by Tang et al. using a bi-layer structure. [22]Between twoelectrodes (indium tin oxide and silver), thin layers of p-typesmall molecules (copper phthalocyanine) and n-type molecules(perylene diimide derivative) were thermally evaporated (vac-uum-processed). The charge separation at the donor-acceptorinterface was found to be very efficient and an impressive

PCE of 1% and high FF of 65% were demonstrated.[

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In theearly 1990s, Heeger, Sariciftci et al.[23a]first discovered electrontransfer from polymer to fullerene, and Yoshino et al. reportedphotoconductivity enhancement when polythiophene isblended with C60buckeyball.[23b]Heeger reported ultrafast pho-toinduced electron transfer (50fs) from a conjugated polymer(MEH-PPV, synthesized by Wudl et al.) to fullerene (C60), whichopened the possibility of using conjugated polymers as thedonor and fullerenes as the acceptor in the OPV devices. [23a]The devices using the bi-layer structure demonstrated lowefficiency,[24] because the lifetime of the exciton is very shortand its diffusion length is only 10 nm.[25,26] This means theactive layer must be very thin to avoid exciton recombination.However, such a thin film (20 nm) is insufficient for effec-

tive optical absorption. To overcome this problem, the bulkheterojunction (BHJ) structure made from a solution of p- andn-type materials was invented by Sariciftci and Heeger in a1992 patent,[27a] followed by two important papers by Heegeret al. and Friend et al., respectively. [27b,c]The nano-scale phaseseparation of the donor and acceptor materials in the bulk cre-ates interfaces for exciton dissociation throughout the film.[27]Encouragingly, much efficiency improvement was achievedby using the BHJ structure. Since then, BHJ has become thestandard device structure in OPV research.

Over the past decade (2003 2013), substantial progress hasbeen made in understanding the working mechanism of the

Yang Yangreceived hisM.S. and Ph.D. in Physicsand Applied Physics from theUniversity of Massachusetts,Lowell in 1988 and 1992,

respectively. Before he joinedUCLA in 1997, he served asthe reseasrch staff in UNIAX(now DuPont Display) from1992 to 1996. Yang is now theCarol and Lawrence E. TannasJr. Endowed Chair Professor

of Materials Science at UCLA. He is an expertise in thefields of organic electronics, such as photovoltaic cells,OLEDs, and memory devices.

OPV devices,[2830]design and synthesis of new materials,[3134]morphology control and characterization,[3538] developingnew device architectures and interface engineering,[3942] andimproving the device stability.[43]As a result, high efficiencies of9% for single junction devices and 11% for tandem deviceshave been published with fairly good stability.[4446]Heliatek Co.in Germany recently demonstrated certified 12% PCE of a vac-uum-processed small molecule organic triple junction tandemsolar cell with an active area of >1 cm2.[47]These are significantmilestones in OPV research. This review article focuses onrecent advances in the aforementioned aspects. Six main topics,including mechanism, new materials, morphology, interfaceengineering & inverted device structure, tandem devices, andmanufacture & stability, are discussed. Future perspectives aregiven with emphasis on new directions derived from current

research and novel applications of OPV technology. Both solu-tion and vacuum processing are important methods for the fab-rication of OPV devices. Thermally evaporated small moleculeBHJ solar cells have achieved 67% in single junction[4850]and beyond 10% in tandem devices.[47,51,52] Impressive workhas been published by Forrest, Leo, Wong, Buerle, and otherson different material systems.[4852] In this review, due to thelimitation of space, our focus is on the solution processed OPVmaterials and devices.

2. Mechanisms

2.1. Basic Structure and Operation of Cell

The typical binary BHJ solar cell is a blend of an electron donorand an electron acceptor. As the solution-coated film dries it phaseseparates into a nanoscale domain morphology that is the key tothe solar cell properties.[38] (The details of Morphology is dis-cussed in the Section-4 of this manuscript.) The basic geometryand energy band configuration are illustrated in Figure1a1c.As indicated in the Introduction part, the optically excited exci-tons in the organic semiconductor are too tightly bound to dis-sociate into free electrons and holes by thermal excitation, [53]but the exciton can dissociate at the interface due to the energy

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in the internal field which has opposite polarity to the externalvoltage, allowing power generation. The usual configuration hasa glass substrate, a transparent metal electrode such as indiumtin oxide (ITO), a hole contact layer, the BHJ active layer and

an electron contact.[

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The inverted device structure swapsthe electron and hole contacts, and provides many advantages.The built-in potential is created by the effective Fermi energy(or work function) of the conducting materials in contact withthe active layer. The hole contact is usually the p-type dopedpolymer PEDOT:PSS or a metal oxide such as MoOX or ITO.Cells that use the metal electrode for the electron contact layerneed to have a low work function such as Ca/Al. However aseparate electron contact layer such as TiOX, can provide thelow work function contact. (In Section-5 of this manuscript,the interface and inverted device architecture are extensivelydiscussed.)

The solar cell current comprises of electrons flowing in theacceptor material and the holes in the donor, which is a unique

feature of the BHJ cell. The carrier mobility and the diffusionlength are both small and current flow is primarily by drift inthe internal field of the built-in potential VBI. The cell thicknessis typically only 100 nm, and the internal space charge createdby the carriers or by residual doping is not enough to give alarge band bending effect, so that the internal electric fieldextends through the active layer.

The PCE of a solar cell efficiency obeys the well-known rela-tion, PCE =VOCJSC FF/Pin, where Pin is the input power.The current in the solar cell at any voltage is most readilyunderstood as the sum of the dark current JD(V) and the photo-current JP(V),

loss associated with the band offset (see Figure 1b). The inter-face band gap EGIis defined as the energy difference betweenthe valence band (or HOMO) of the donor and the conductionband (or LUMO) of the acceptor. Provided that EGIis less than

the exciton energy, it is energetically favorable for the exciton toseparate at the interface into a hole in the donor and an electronin the acceptor. This is known as the charge transfer (CT) stateand mobile free electrons and holes are formed from this state.

An important unresolved question is how small an energydifference between the exciton energy and EGI can maintainefficient charge separation, since the energy difference rep-resents a significant loss of cell efficiency.[54] Also unclear iswhether the excitons reach the interface by diffusion or byenergy transfer but, in either case, their range is only a few 10 sof nanometers.[55]Hence, the BHJ structure must have a com-parable nanoscale structure to be an effective solar cell. Fortu-nately, phase separation with domain size about 20 nm occursspontaneously in a range of materials. The exciton reaches the

BHJ interface within 1 ps,[56]and the exciton radiative recombi-nation is almost completely suppressed.

An efficient organic solar cell therefore requires strongabsorption of light to form excitons, followed by efficienttransfer of the excitons to the CT state at the interface, dissocia-tion of the CT state into mobile free electrons and holes, andfinally a high probability of charge collection at the contacts.The formation and dissociation of the CT state is perhaps themost complex and controversial process, and this is discussedafter some other concepts are introduced.

As in other solar cells, the BHJ cell is constructed with abuilt-in potential VBI so that the electrons and holes separate

Figure 1. Basic geometry and electronic stricture of the BHJ cell. (a) The nanoscale phase-separated domain morphology showing exciton absorptionleading to the charge transfer state that separates into mobile electrons and holes. (b) The same process illustrated in an energy diagram showing theband offset between the donor and acceptor materials. (c) Schematic of solar cell current flow at voltage bias V. (d) Example of cell current-voltage char-acteristics (points) and the same result obtained by separately adding the dark and photocurrent (solid line). The dashed line illustrates the expectedcell current in the absence of significant recombination. (e) Examples of dark forward bias current for various organic cells.

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is constant up to VBI, representing an approximation to an idealcell, with fill factor is 0.8. The lower fill factor in real cells resultsfrom recombination of electrons and holes before they reach thecontacts and is discussed in section 2.5.

2.2. Electronic Excitations in BHJ Cells

The different electronic states represented by excitons, the CTstate, the mobile transport states and the localized trap stateseach play a crucial role in the operation of an organic solarcell. These states have characteristic optical absorption spectrawhich are illustrated in Figure2a.[61]The excitonic absorptionis strong with a series of absorption bands that are sensitiveto the underlying order of the material. [62] The absorption ischaracterized by strong electron-phonon coupling that givesrise to phonon sidebands that are often obvious in the spectra

J(V) = JD(V)+ JP(V) (1)

In the solar cell operating voltage range, the dark current andphotocurrent have opposite signs. A typical cell current-voltageJ(V) characteristic is illustrated in Figure 1d and confirms thevalidity of Equation (1). VOC is the voltage at which JD(VOC)

and JP(VOC) are equal and opposite. The photocurrent changessign when the applied voltage exceeds the built-in potential,and hence VOC< VBI. By definition, no current flows at VOC,and therefore the carrier excitation rate is balanced by an equalcarrier recombination rate. The excitation rate G under 1-sunillumination in a typical organic cell is about 1022 cm3, andthe recombination lifetime is about 106sec, from which thecarrier concentration of both electrons and holes, NC= G isabout 1016cm3.[58]At short circuit, for a material with mobility103cm2/Vs and cell current 10 mA/cm2, the carrier concentra-tion is about 1015 cm3. These approximations give the orderof magnitude of carrier concentration, which increases towardsVOCas the internal field decreases.

The carrier concentration also determines the electron andhole quasi-Fermi energies EqFE, EqFHthrough,

NC= N0exp[(EC Eq F E,H)/kT] (2)

where N0 is the effective density of states at the band edge.From the estimates for NC at VOC, the quasi-Fermi energiesare about 0.25 eV from the band edges. VOCcannot exceed EGI-EqFE-EqFH, otherwise current will flow in the forward direction.Hence VOCis set by whichever is smaller of the built-in poten-tial or the quasi-Fermi energies and for the latter case, VOCexpected to be about 0.5 V less than EGI, which is confirmed bymeasurement.[59]Equation (2) applies to a semiconductor witha sharp band edge and should be generalized when there is aband tail of localized states. However, the estimate of the quasi-Fermi energy is not changed much for the band tail in typicalorganic cells.

Figure 1e shows examples of the dark forward current insome solar cells, which obey the well-known diode equation,

JD(V) = J0exp

e(V RSJD)

nkT

1

+

V RSJD

RP (3)

where n is the ideality factor, RP is the parallel shunt resist-ance and RS is the series resistance. The pre-factor J0 isrelated to the barrier height and decreases with larger VBI.The shunt resistance is highly variable in different devicesand its precise origin is unclear. The series resistance canarise from the specific contacts or from within the BHJ active

layer. Large series resistance significantly degrades the cellJ(V) characteristics.

The ideality factor is typically in the range 1.5-2 in goodorganic cells. Classic semiconductor theory shows that n = 1 isassociated with band-to-band recombination while n =2 is associ-ated with recombination through states in the middle of the bandgap by the Shockley-Read-Hall mechanism. Modeling shows thatthe intermediate values of n which are observed in other disor-dered semiconductors, arise from recombination through bandtails states, possibly in combination with deeper states.[60] Thedashed line in Figure 1d illustrates the expected cell J(V) whenthe dark current has an ideality factor of 1.5 and the photocurrent

Figure 2. (a) Optical absorption of some BHJ cell component mate-rials, PC61BM and two different polymers with the indicated structure.(b) Photocurrent spectral response from PCDTBT:PC61BM showing theweak absorption from the CT state and from deep traps.

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this is not the case at least in P3HT:PC61BM cells.[68]A morerecent model of assisted ionization comes from the evidencefor some intermixing of the fullerene into the polymer.[69]Theidea is that intermixing creates an interface zone where it isenergetically favorable for the electron and hole to separate fur-

ther, leading to efficient ionization.

2.3. Mobile States, Localized Band Tail States and Dispersive

Transport

Electrons and holes that are ionized from the CT state movein the internal field to the cell contacts. The cell materialsare disordered which is known to introduce a band tail oflocalized states extending into the band gap, as illustrated inFigure3a3c, and polaron behavior adds to the tendency of car-riers to localize. The mobility edge model describes a situation

and are split by about 0.18 eV, the energy ofvibrational modes of the conjugated ring.A more ordered (i.e. semicrystalline) mate-rial morphology tends to have narroweroptical absorption bands shifted to a lowerenergy than a more disordered form. The

excitations are essentially molecular, beingprimarily confined to a single polymer mol-ecule. The low dielectric constant leads to alarge Coulomb binding energy that tightlyconfines the electron and hole.

The CT state comprises an electron andhole on either side of the BHJ interfaceand therefore has an intermediate characterbetween the exciton and the free carriers,because the electron and hole separationreduces the Coulomb interaction.[63] TheCT optical absorption is weak because itonly occurs at the interfaces, but it can beobserved by various sensitive techniques,

like the photocurrent spectral response(PSR) which is shown in Figure 2b.[64] TheCT absorption is a broad featureless bandextending below the exciton absorptionbands with no sign of phonon sidebands.The CT state is an excitation across the inter-face band gap (see Figure 1) and so providesa measure of EGI. Street et al. have proposedthat the shape of the CT absorption reflectsthe joint density of states of the polymervalence band and the fullerene conductionband, while Vandewal et al. propose that thebroadening is primarily due to an electron-

phonon interaction.[

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]

The CT state dissociates easily into mobilefree carriers in the higher efficiency organicsolar cells. The reason for the high disso-ciation probability has been much discussedand the explanations fall into three generalcategories. One explanation is ionization ofthe CT state by the internal electric field, awell-known process described by the Braun-Onsager model.[66]This mechanism not only explains the crea-tion of mobile carriers but also how the geminate recombina-tion of the CT state at low internal field can account for thereduced FF of the cell. However, although this mechanismdoes occur in some cell materials, it is insignificant in at least

some of the higher efficiency cells.[67]A second explanation isthat the binding energy of the CT state is sufficiently small sothat thermal ionization occurs with high probability even at lowinternal field. This mechanism is estimated to occur when thebinding energy is less than about 0.25 eV, which is roughly theexpected binding energy of the CT state.[58]

The third explanation is assisted thermal ionization, whichinvokes an additional mechanism to push the CT state furtherapart and make complete ionization easier. An early suggestionof this type was that the excess kinetic energy of the separatingelectron and hole pushes the carriers apart so that they neverreach the CT ground state. However, measurements show that

Figure 3. (a) Illustration of the expected density of states in a disordered semiconductor withband-like states, localized band tails and deep traps. (b) Multiple trapping of carriers in a bandtail/mobility edge model. (c) transient photocurrent data for PCDTBT:PC61BM showing dis-persive transport characteristic of the band tail/mobility edge model. (d) Photocurrent spectralresponse measurements showing the increase in low energy absorption as a result of illumina-tion in PCDTBT:PC61BM and P3HT:PC61BM. (e) Atomic model of polythiophene illustratingthe hydrogen vacancy and CH2model of defect formation. (c) Reproduced with permissionfrom.[74](d,e) Reproduced with permission from.[77]Copyright 2012 American Physical Society.

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illumination, as measured by the low energy absorption bandin a PSR measurement, and are implicated in the loss of effi-ciency of some solar cells exposed to prolonged sunlight.[77]

Recent evidence suggests that the light-induced deep states

arise from the breaking of C-H bonds and the migrationof the hydrogen to other sites on the polymer or possibly onthe fullerene.[77] The site vacated by H and a site with excesshydrogen are illustrated in Figure 3e and are defects capableof forming a trap, as has been shown by first principle calcula-tions. The states formed by illumination by white light, UV lightor soft x-rays are reversible by annealing at about 100 C andwith an activation energy of 11.2 eV.[77]Again, theory showsthat the migration of hydrogen within the polymer occurs withan energy of 1.21.4 eV, consistent with the annealing data andproviding further evidence for the hydrogen bond breakingmodel of these deep states.

2.5. Recombination Mechanisms

Recombination of the electron and hole competes with collec-tion of the charge at the electrode and is one of the primaryreasons for low solar cell efficiency. Carrier recombinationincreases as VOC is approached because the internal elec-tric field decreases and hence the rate of charge collectiondecreases. The four main recombination mechanisms that areanticipated in organic solar cells are illustrated in Figure 4.Recombination mechanisms are characterized by their recom-bination order. First order mechanisms are proportional to thefree carrier concentration, while second order recombination

where conduction occurs in the region of higher density ofstates but not in the deeper localized states.[70] The dividingline between localized states and mobile states may be at a well-defined energy or more gradual, but does not greatly affect themodel. This mobility edge model accounts well for the proper-ties of organic thin film transistors. [71]

The band tail/mobility edge model is characterized by lowaverage mobility because carriers are repeatedly trapped inlocalized states and excited to the mobile states (Figure 3d) sothat the effective mobility is expressed as Equation (4),[70]

: = : 0tF

tF + tT(4)

where 0 is the free carrier mobility and tF and tTare the rel-ative time that the carrier is free or trapped, respectively. Anexponential band tail has the property that as time progresses,carriers are increasingly likely to be trapped in rare deep bandtail states and take much longer to be thermally excited. Hencethe mobility of an excited carrier decreases with time, a prop-erty known as dispersive transport.[72] Transient photocon-ductivity in which a short pulse of laser illumination excitescarriers which move across the sample is the classic experi-ment to observe the effect. The data in Figure 3c confirm thepower law decreases in the current which reflects the timedependent mobility, t1, and which changes to a steeperpower law after the transit time. This result is characteristic ofdispersive transport from an exponential band tail, described bya density of states, NBT(E) = N0exp((E-E0)/EB), and the slopeof the exponential is EB= kT/.[73]These measurements findEB= 35 meV and 45 meV for the polymer in P3HT:PC61BMand PCDTBT:PC61BM respectively,[74]and the same exponentialslopes are observed in PSR data as in Figure 1e.

The trap-limited mobility of the polymer in a typical organic

solar cell is 103104 cm2/Vs. It is not easy to deduce themobility of the free carriers but it is probably 1 cm2/Vs orlarger. Solution cast organic thin film transistors have mobilityup to 110 cm2/Vs and single crystal organic have mobility>10 cm2/Vs. Mobile electron and hole states are evidently sig-nificantly delocalized, and there is further evidence for delocali-zation in BHJ cells.[75]Excitons and mobile carriers apparentlyhave different characteristics, the former being highly molec-ular and the latter being relatively delocalized. The contributionof the electron-phonon coupling to the exciton energy is clear,but the magnitude of phonon coupling in the mobile carrierstates is less obvious, and is probably a strong function of thestructural disorder.

2.4. Deep Localized States

Almost every type of solar cell is limited by deep localized statesthat act as traps and recombination centers and organic solarcells are no different. In crystalline silicon it is surface states,in amorphous silicon it is dangling bonds and in polycrystal-line materials it is usually grain boundaries. A small den-sity of traps, of order 101516 cm3, is sufficient to impact theefficiency of a cell. Localized states in organic BHJ cells havebeen observed by a variety of measurement techniques. [76]Figure 3d shows that the deep states can be enhanced by strong

Figure 4. Illustration of the main recombination mechanismsexpected in BHJ organic cells; (a) geminate recombination; (b) mobilecarrier recombination; (c) deep state recombination; (d) reverse diffusion.

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The starting point for any model is the density of statesdistribution, which as we have shown comprises of mobilestates at the band edges, localized band tail states that limitcarrier mobility and deep localized states that can act asrecombination centers. An example of the DOS is shownin Figure 5a for PCDTBT:PC61BM, and is based on theo-

retical calculations of the density of states in the bands,measurements of the band tail slopes and estimates of thedefect density, along with some interpolation, where thereis incomplete data.[74] The model combines the HOMOstates of PCDTBT and the LUMO states of PC61BM whichare the relevant states for the cell. Although small in totalnumber, the majority of carriers occupy the band tail states(see Figure 5a) and relatively few are mobile. Recombinationis probably dominated by mobile carriers which can moveto a recombination center, rather than by localized band tailstates which can only tunnel a very short distance. The DOSalong with the specific recombination mechanisms and thebuilt-in potential and other parameters, allows a numericalsolution of the drift-diffusion equations. An important role

of modeling is to predict how the performance of cell canbe improved. For example, Figure 5b shows a result of anempirical model to estimate how the solar cell efficiencydepends on the band offset energy, and shows that if theband offset could be almost eliminated, then cells that arenow about 12% efficient, can increase to above 20%. Theresult motivates the search for materials with low bandoffset.

3. New Materials

There has been intense research on developing novel organic

semi-conducting materials, including polymers and smallmolecules (SM), for BHJ photovoltaic devices during thelast decade. Here, some representative p-type and n-typematerials featuring several important design rules will bereviewed. Their chemical structures are shown in Figure 6(p-type) and Figure 7 (n-type). For the p-type materials dis-cussed here, the corresponding n-type material is PC61BM orPC71BM.

3.1. P-type Materials: Polymers

Conjugated polymer based electron donor mate-rials are the best-studied materials to date forthe OPV. Among them, poly(3-hexylthiophene)

(P3HT) is the most commonly used materialdue to the advantages of easy synthesis, highcharge carrier mobility, good processability, etc.Highly regioregular P3HT can be obtained viaMcCullough, Rieke or GRIM methods.[8385]Efficiencies of 4 5% based on the regioreg-ular P3HT have been achieved about ten yearsago.[8688] Since then, numerous groups havebeen trying to increase the performances bymodifying the morphology, the device archi-tecture, and the electron acceptor. As a result,efficiencies can now go up to 7% with external

is proportional to the square of the free carrier concentration.First order recombination results in a cell FF that changes littlewith illumination intensity, while second order recombina-tion results in a FF that decreases as the illumination intensityincreases.

Geminate recombination (Figure 4a) is a first order mech-

anism which occurs when the CT state recombines beforethe electron and hole dissociate.[66] As discussed in 2.2, fielddependent dissociation competes with recombination and canexplain J(V) in some cells but is absent in others. Mobile freecarrier (Langevin) recombination (Figure 4b) is the recombi-nation of mobile electrons and holes, [78]and is a second-orderprocess that can be identified and quantified from the light-intensity dependence. Recombination occurs through the CTstate and so is closely related to geminate recombination. Thismechanism usually dominates at sufficiently high light inten-sity and hence the primary question is its relative strength atsolar intensities compared to other mechanisms. Localizedstate recombination (Figure 4c), for example between a mobilehole and a trapped electron,[79]is an important mechanism best

identified by correlating the deep state density to the recom-bination. For example, deep states introduced by prolongedillumination are correlated to a corresponding increase inrecombination.[77]Reverse diffusion to the contact (Figure 4d)occurs when an exciton splits close to one of the electrodes, andthe wrong carrier diffuses against the internal field to reachthe contact and recombine.[80] This is usually a small contri-bution to recombination but can become significant if othermechanisms are suppressed.

2.6. Modeling of Cell Properties

Although simple in concept, solar cells are complex in detailand in a disordered material typically requiring numericalmodeling to fully analyze.[81] A caution is that cell modelsoften ignore the effects of cell contact resistance, which sig-nificantly changes the J(V) characteristics.[82] There is plentyof evidence for a series contact resistance in organic cells, andalso for contact degradation when some cells are exposed to theambient.

Figure 5. (a) Density of states distribution for PCDTBT:PC61BM showing mobile and localizedband tail states. The occupancy under 1-sun illumination is shown. (b) Empirical model of thecell efficiency as a function of the interface band gap and the band offset. Reproduced withpermission from.[74]Copyright 2011 American Physical Society.

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One of the successful examples at theearly stage is the carbazole and benzothiadia-zole (BT) based polymer (PCDTBT) reportedby Leclerc et al.[93] The optical bandgap ofthis polymer is 1.88 eV with a low-lyingHOMO energy level of 5.50 eV. Initial

results of the photovoltaic devices showeda high VOCof 0.89 V, a JSCof 6.92 mA/cm2,a FF of 63%, and a PCE value of 3.6%.Later, Heeger et al. introduced a solutionprocessed TiOx as the electron transportlayer and optical spacer in the device andachieved a certified PCE of 6.1% with higherJSCover 10 mA/cm2and high FF of 66%.[94]Recently, a PCE of 7% has been achievedby device engineering, demonstrating thegreat potential of the carbazole based poly-mers.[95]However, the bandgap of PCDTBTis still too large to harvest photons in theNIR region. To further lower the bandgap,cyclopenta[2,1-b;3,4-b]dithiophene (CPDT)and dithieno[3,2-b:2,3-d]silole (DTS) unitswith stronger electron-donating propertieswere independently synthesized by Yanget al. and Brabec et al., respectively.[96,97]The polymers PCPDTBTand PSBTBTshowsmall bandgaps of 1.5 eV due to the strongintra-molecular donor-acceptor interactions.Interestingly, the performance of PCPDTBTbased devices can be enhanced significantlyby adding 3% 1,8-diiodooctane (DIO) as aprocessing additive to tune the morphology,whereas PSBTBT does not.[98] Both of them

show PCE 5% in a single junction devicewith photo-response up to 850 nm for PCP-DTBT and 820 nm for PSBTBT. The majordifference is that the C-Si bond is longerthan the C-C bond. Consequently, less sterichindrance is created in the DTS core by theside chains, leading to a better stacking

and enhanced crystallinity.[99] Another important buildingblock is diketopyrrolopyrrole (DPP), which was first used inOPV in 2008 by Janssen et al.[100] The electron-withdrawingeffect of the lactam units causes the chromophore to havea high electron affinity and thus, it can be used as a strongelectron-withdrawing unit. When polymerized with a thio-phene unit via Suzuki cross-coupling, a low bandgap polymerPDPP3Twas reported by Janssen et al. in early 2009. [101]Thepolymer shows bandgap as low as 1.31 eV and deep HOMOlevel of 5.17 eV. This low bandgap polymer showed a photo-response up to 930 nm when combined with PC71BM, andEQE was around 35%. Good photovoltaic performance of 4.7%PCE with VOC=0.65 V, JSC=11.7 mA/cm2, and FF =0.60 wasachieved. Very recently, by increasing the molecular weight ofPDPP3T and improving the thin film morphology, PCEs over6% have been achieved.[102,103]These three polymers representan important family of photovoltaic materials due to their rel-atively small optical bandgaps, which are useful in a tandemsolar cell device.

quantum efficiencies around 70%.[8992]However, the main issuewith P3HT is its quite large bandgap (1.9 eV) and high HOMOlevel, which lead to insufficient NIR photon absorption and lowopen circuit voltage of the corresponding devices. To harvest agreater part of the solar spectrum and provide as much VOCas pos-sible, narrowing the bandgap and down-shifting the HOMO levelof the p-type materials (or up-shifting the LUMO level of the n-type

materials) have become important topics for materials scientists.To realize these goals, several synthetic strategies have been

developed and proven to be very effective: (1) construct thebackbone using alternating electron-rich (donor) and electron-deficient (acceptor) units to form the D-A co-polymers; (2) sta-bilize the quinoid resonance structure; (3) incorporate strongelectron withdrawing substitutes such as carbonyl group or flu-orine atoms; (4) attach conjugated side chains on the polymermain chains. The details can be found in recent reviews on thechemistry of the materials.[3134]By using or combining theseapproaches, numerous new compounds have been designedand synthesized for OPV applications.

Figure 6. Chemical structures of some high performance p-type materials including conjugatedpolymers and small molecules.

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A breakthrough in new material and high efficiency singlejunction OPV device was achieved by joint efforts between Yu(U. of Chicago) and Li (Solarmer Energy Inc.). Yu first reportedthe thieno[3,4-b]thiophene (TT) building block, which can sta-bilize the quinoid structure to reduce the bandgap.[104] Yanget al. reported the application of the benzo[1,2-b;4,5-b]dithio-phene (BDT) unit in OPV materials. [105]When combining thesetwo units together and adding a fluorine atom on the TT unit

to lower the HOMO level, from 2009 to 2010, Yu, Li and Houet al. reported several high performing polymers, such as PTB7and PBDT-TT-CF.[106,107]They have similar bandgap of around1.6 eV and HOMO level of around 5.2 eV. The initial resultswere reported as 7.4% and 7.7% PCEs in single junction devicesfor PTB7 and PBDT-TT-CF, respectively. Recently, Hou et al.reported several modification on this family (PBDTTT-C-T andPBDTTT-S-T) with with conjugated thiophene side chains onBDT unit.[108]Note worthy, this series of polymers set the mile-stones of 7%, 8%, and even 9% PCEs, respectively (details willbe discussed in the interface section), which greatly boosted themomentum in OPV field. These polymers are still among the

leading figures in single junction devices up to date.[45]Despitethe high efficiency of the BDT-TT based polymer, the tedioussynthesis of the fluorinated TT monomers does not make thecost very low. Later, another easily-synthesized strong electronacceptor unit, thieno[3,4-c]pyrrole-4,6-dione (TPD), was reportedby a number of groups. The first polymer, PBDT-TPD, was pub-lished in 2010 by the Leclerc, Frechet, and Jen groups indepen-dently.[109111] The polymer shows a bandgap of 1.81 eV and

very deep HOMO level of 5.57 eV. The initial device resultsdiffer from each group. The best one by Frechet et al. gave aJSCof 11.5 mA/cm2, a VOCof 0.85 V, a FF of 0.68 and a PCE of6.8%. To lower the bandgap of the TPD based polymers, Tao etal. copolymerized it with the DTS unit and obtained a polymerPDTS-TPDwith bandgap of 1.73 eV and deep HOMO level. [112]Higher PCE of 7.3% was achieved mainly due to a higher JSCcompared to the PBDT-TPD based devices. Further optimi-zation on this structure was reported by Reynolds et al., whoreplaced the silicon atom in the DTS unit with a germaniumatom to form the dithienogermole (DTG) unit.[113] The newpolymer PDTG-TPDshows slightly lower bandgap of 1.69 eV,

Figure 7. Chemical structures of some high performance n-type materials including fullerene and non-fullerenes derivatives.

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and Yoshinura et al. reported a new family of low bandgap poly-mers using an asymmetric electron rich dithieno[3,2-b:2,3-d]pyran (DTP) unit.[128]The electron-donating property of the DTPunit was found to be the strongest among the most frequentlyused donor units, such as BDT, DTS or CPDT. When the DTPunit was polymerized with the strongly electron-deficient DFBT

unit, a regiorandom polymer (PDTP-DFBT, bandgap =1.38 eV)was obtained. It was found that the DTP based polymer PDTP-DFBT shows significantly improved solubility and processa-bility compared to the BDT or CPDT based polymers with samealkyl side chains. Consequently, very high molecular weightand soluble PDTP-DFBT can be obtained with less bulky sidechains. PDTP-DFBT shows excellent performance in bulk-heterojunction solar cells with power conversion efficienciesreaching 8.0% (VOC= 0.69 V, JSC= 18.0 mA/cm2, FF = 0.64),with EQE over 60% in the NIR region.[46,128]As a result, tandempolymer solar cells fabricated with P3HT and PDTP-DFBTachieved 10.6% efficiency as certified by NREL.[46,128]

As we can see, through smart materials design, differentfamilies of polymers with higher and higher PCEs can be

obtained. Aside from the new structure design, it is very impor-tant to note that polymer purity, molecular weight, the poly-dispersity index, and the choice of solubilizing side chain arealso essential for determining performance. It is obvious thathigher purity will lead to less defects and thus better chargetransport in the bulk. Although the purity-performance depend-ence of a polymer solar cell is not as high as a silicon solar cell,it is still important to keep the impurity level as low as pos-sible (at least


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6.7% in 2011 was achieved using DTS(PTTh2)2:PC71BM blend.Notably, this high effciency was obtained by adding remarkablylow percentage of solvent additive (0.25% v/v of DIO) duringthe film-forming process, which led to decreased domain sizesin the BHJ layer.[142] Bazan et al. carried out a detailed studyon this series of molecules and found that the shape, bending

angle, and dipole moment (the position of the nitrogen atomon the PT unit) of the molecules affect their performance dra-matically.[143]Recently, Gupta and Heeger et al. reported a newmaterial, DTS(FBTTh2)2,by replacing the PT unit with a fluori-nated BT unit to lower the HOMO level and enhance the VOCofthe devices.[144]Together with a polyethylenimine (80% ethoxy-lated) (PEIE) modified ZnO electron transporting layer in theinverted device, an amazing PCE of 7.88% was achieved with aJSCof 15.2 mA/cm2, VOCof 0.77 V and a FF of 0.67. In addition,the devices based on DTS(FBTTh2)2 show high stability whenstored in air.[144]This rapid progress in the solution processedSM BHJ solar cells has significant influence in the OPV fieldand therefore, more exciting improvements in the near futurecan be anticipated.

3.3. N-type Materials

The n-type semi-conductor plays an equally important role asthe p-type material in a solar cell. [145,146]In 1995, Heeger et al.invented the BHJ structure for the polymer solar cells incorpo-rating a soluble C60 derivative PC61BM (synthesized by Wudlet al.) as an acceptor blended with conjugated polymer donor.[27]Since then, PC61BM and its corresponding C70 derivativePC71BM (first reported by Janssen et al.), [147]which possessesstronger visible absorption than PC61BM, are widely used inthe fabrication of BHJ OPV devices. Although great efforts

have been made in developing new n-type materials in the lastdecade, including fullerene derivatives and non-fullerenes, it isquite interesting that PC61BM/PC71BM is still the best choicefor most of the high performance polymers.

So far, most of the work on fullerene derivatives is aiming atup-shifting the LUMO level of the acceptor to enhance the VOCof the solar cell device. An example are the trimetallic nitrideendohedral fullerenes, which were discovered in 1999 by Ste-venson and coworkers.[148]Theoretical and experimental studiessuggest that the LUMO energy levels of this type of fullerenesare much higher than those of their corresponding empty-cagefullerenes. Recently, Drees et al. synthesized a series of solublePC61BM-like Lu3N@C80 derivatives. Among them, Lu3N@C80-PCBHshows similar solubility and miscibility to PC61BM.

The LUMO energy level of Lu3N@C80-PCBH was found to be0.28 eV higher than that of PC61BM. The electron mobility ofLu3N@C80-PCBH was measured to be 4.0 104 cm2V1s1compared to 1.4 103 cm2V1s1 for PC61BM. The solar celldevices based on P3HT:Lu3N@C80-PCBH displayed VOC of0.89 V, which is 0.26 V higher than that of the PSC based onP3HT:PC61BM and similar JSC and FF values as those of thePSCs based on P3HT:PC61BM. The maximum PCE of thedevices based on P3HT:Lu3N@C80-PCBH reached 4.2%.[148]However, the high synthetic cost of this series materials maylimit their real application in commercial products. Anotherapproach to lift the LUMO level of the fullerenes is to make

in the last few years.[133135]Compared to the polymeric coun-terpart, small molecules are expected to have higher molecularprecision relative to the statistically determined nature of syn-thetic polymers and less batch to batch variations. Therefore,more advantages, such as reproducible fabrication protocols anda better understanding of structure property relationships, are

anticipated.[

136138

]

While a library of oligothiophenes has beenbuilt and explored over the past two decades, donor-acceptortype small molecules appended with solubilizing substituentsare now being systematically studied as more promising can-didates for OPV application. So far, a variety of structures havebeen reported, such as subphthalocyanine, merocyanine, squar-aine, diketopyrrolopyrroles, borondipyrromethene, isoindigo,perylene diimides, fused acenes, oligothiophenes, and triph-enylamine derivatives.[136138]Several kinds of molecular shapeand arrangements, like linear molecules with D-A, D-A-D orA-D-A structures and three-arms, X-shape or star-shape mol-ecules, have been reported.[133138]

Early efforts on solution process small molecule OPVshowed that the efficiency was limited by the low photo-current

and fill factor and the initial PCEs were only 13%. An encour-aging discovery was reported by Nguyen et al. in 2009. Theyshowed that a DPP based SM, DPP(TBFu)2, can give up to 4.4%PCE in a BHJ device.[139]It was found that when DPP(TBFu)2isblended with PC71BM, very little phase separation was apparentin the as-cast film. Interestingly, thermal annealing of the filmled to suitable phase separation so that favorable BHJ mor-phology was obtained. The degree of phase separation can becontrolled by adjusting annealing temperature; it was foundthat 110 C annealing yielded optimum device properties with aJSCof 10 mA/cm2, a VOCof 0.9 V, and a FF of 0.48. [139]Encour-aged by this work, more and more recent efforts have beendevoted to the development of new small molecular materials

for OPV application. Chen and coworkers designed and syn-thesized a series of oligothiophenes end-capped with electron-withdrawing alkyl cyanoacetate groups.[140] The alkyl cyanoac-etate end group can reduce the HOMO level of the moleculeand improve the film quality significantly. The best performingmolecule, DCAO7T, exhibited a bandgap of 1.8 eV and PCE of5.08%, with a JSC of 10.7 mA/cm2, VOCof 0.86 V, and a FF of0.55 as reported in 2011.[140]Recently, the cyanoacetate groupwas replaced with a 3-ethylrhodanine moiety as the end acceptorto narrow the bandgap and further reduce the HOMO level ofthe SM; the thiophene unit in the middle of the molecule wasreplaced by a co-planar BDT unit to enhance the packing. A newmolecule, DR3TBDT(bandgap 1.7 eV), was published with ahigh PCE of 7.4% with a JSCof 12.2 mA/cm2, VOCof 0.93 V, and

a FF of 0.65.[141]All the parameters are higher than DCAO7Tbased devices due to the smaller bandgap, deeper HOMO level,and better thin film morphology. Another important family ofSM was innovated by Bazan and Heeger et al., which has a D-A-D-A-D structure. The most studied molecule, DTS(PTTh2)2,consists of an electron donating DTS unit as the middle core,two strong electron withdrawing [1,2,5]thiadiazolo[3,4-c]pyri-dine (PT) units, and two alkylated bithiophenes as the endgroup.[142] The material exhibited strong optical absorptionfrom 600 to 800 nm and a high hole mobility of 0.1 cm2V1s1measured by organic field-effect transistor. Under AM1.5 irra-diation (100 mW/cm2), a record SM based BHJ device PCE of

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0.76 V and 1.86%, respectively.[157]More recently, by changingthe thiophene unit on the FFI-1 to a 2-methylbenzene group,higher VOCand JSCwere observed in P3HT based devices andan impressive PCE of 2.9% was achieved.[158] Polymer basedelectron acceptors have also been explored for several years.Relatively well studied examples are F8TBT, CN-PPV and

PDI-DTT.[

159161

]

F8TBT and PDI-DTT were used in BHJ solarcells blended with P3HT and CN-PPV was used in a bi-layerdevice structure. PCE 2% for F8TBT, CN-PPV and PDI-DTTbased devices were reported by McNeill et a., Friend et al., andZhan et al., respectively. Although the VOCwas higher than thePC61BM based devices, the substantially low JSC and FF lim-ited the efficiency greatly.[159161]The electron mobility does notseem to be a problem. Therefore, a better understanding of theprofound interactions between acceptor and donor materials,especially in blend systems, will undoubtedly play a criticalrole in improving the performance of OPV devices using non-fullerene acceptors.

4. Morphology

In BHJ solar cells, the morphology of the PV active film is crit-ical for the achieving high efficiency. In organic/polymer mate-rials, while the atoms are well organized within the molecules,the weak Van der Waals force between molecules results in arelatively disordered arrangement. As discussed in Section-2,the excitons in organic/polymer materials are strongly localizedFrenkel excitons. In the simplified photophysics model, uponphotogeneration of excitons, the large exciton binding energy(>>kT at room temperature) prohibits excitons to be dissociatedthermally and under normal electric field, and this task needsto be accomplished at the donor/acceptor interface. The much

lower carrier and exciton mobility in the organic materials pre-vents the planar junction structure used in inorganic solar cellto be applied to the OPV devices. The preferred morphology ofBHJs is a bi-continuous interpenetration network. Both donorand acceptor domains should have the sizes of twice the excitondiffusion length (10 nm), so that excitons can diffuse to theD-A interface to achieve high exciton diffusion and dissociation(charge separation) efficiency to generate charge. After chargeseparation at the D-A interface, holes and electrons need totravel to positive and negative electrodes through donor andacceptor networks, respectively.

4.1. Thin Film Morphology Investigated via AFM, SEM and TEM

The understanding of polymer solar cell morphology hasgone a long way and is strongly related to materials systems.One of the early successful polymers for solar cells is poly(2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene),MDMO-PPV. In 2001, Shaheen et al. showed that in a MDMO-PPV:PC61BM system, the organic solvent selection plays impor-tant role.[162]Using chlorobenzene (CB) as the solvent led to amuch smoother polymer blend film than that using toluenesolvent. Figure 8a, 8b show the AFM images of the MDMO-PPV:PC61BM films cast from toluene and CB, respectively.The device performance exhibits a significant improvement,

a bisadduct on it. Indeed, a number of fullerene bisadductsbased on PC61BM (Bis-PC61BM) and related structures weresynthesized and higher VOCand PCE were obtained for P3HTbased devices.[149,150]In 2009, Li et al. reported a new fullerenebisadduct, indene-C60 bisadduct (IC60BA), via a simple one-pot reaction of indene and C60.[151]The product is a mixture of

unreacted fullerene, indenefullerene monoadduct, indenefullerene bisadduct and indenefullerene mutiadduct. Eachcomponent can then be separated and purified by silica gelchromatography. Surprisingly, when IC60BA was blended withP3HT and applied into solar cell devices, PCEs of 6.5% wereachieved with a JSCof 10.6 mA/cm2, a high VOCof 0.84 V, anda FF of 0.72, which is the highest value for a P3HT based solarcell device.[152] Except for the higher LUMO level of IC60BA,the beauty of this molecule is that it does not affect the chargetransport when blended with P3HT. IC60BA was then blendedwith other low bandgap polymers; nevertheless, the attemptswere not very successful.[153]Lower JSC and FF were obtainedin most cases, probably due to insufficient LUMO-LUMOoffset between donor and acceptor materials for efficient charge

separation or due to poor thin film morphology. To make theindene-fullerene adduct more compatible with low bandgappolymers, Yang et al. designed and synthesized indene-C70monoadduct with an ester group on the indene unit (the syn-thesis of bisadduct was not successful). The new fullerenederivative, H120, was tested using PBDT-TT and PBDTT-DPPas the donor material. Slightly higher VOC and similar PCEcompared to PC61BM based devices were obtained.[154]Anotherinteresting fullerene acceptor is SIMEF, which was synthesizedby Nakamura et al. in 2009.[155]It was reported that SIMEF has0.1 eV higher LUMO level than that of PC61BM. The sameauthors fabricated the solution processed pin photovoltaicdevices based on a tetrabenzoporphyin derivative as the donor

and SIMEF as the acceptor. After device optimization, the VOC,JSC and PCE of the device reached 0.75 V, 10.5 mA/cm2 and5.2%, respectively, while the VOC, JSC and PCE of the deviceusing PC61BM as acceptor were only 0.55 V, 7.6 mA/cm2 and2.0%, respectively.[155]

Although fullerene derivatives show very promising proper-ties for OPV application, their relatively high cost is a concern.The pursuit of high performance, non-fullerene acceptors havedrawn great attention in recent years.[146]Both small moleculesand polymers with high electron affinity have been investi-gated to find an alternative for PC61BM/PC71BM. Besides thelower cost, an important feature for the non-fullerene accep-tors is that the high absorption coefficient in the visible regioncould be achieved and thus, they can compensate the absorp-

tion range of low bandgap polymers.[146] For example, Wudlet al. synthesized 9,9-bifluorenylidene (9,9-BF) as a scaffold forsmall molecule acceptors.[156]The 9,9-BF structure experiencestorsional strain in the ground state due to repulsive interactionbetween H1H1 and H8H8 protons, but the strain can berelieved when the molecule receives an extra electron andgains aromaticity. Initial characterization of BHJ solar cellswith P3HT has revealed a particularly high VOC= 1.1 eV anda respectable FF = 40%, yielding PCEs as high as 2%. [156] Peiet al. reported a series of fluoranthene-fused imide (FFI) deriva-tives. The first molecule, FFI-1, when used as the acceptormaterial for P3HT, the VOC and PCE of the devices reached

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more efforts are needed to replace them withenvironmental friendly solvents such as non-halogenated solvents.

Regioregular (RR) P3AT has high regioreg-ularity and enabled high crystallinity andhigh mobility (0.1 cm2/v-s),[165]which in turn

led to intensive interest in organic electronicssociety on organic transistors and OPV. Withhigh mobility and much improved absorp-tion comparing to earlier generation PPVpolymers, RR-P3AT (particularly RR-P3HT)should easily lead to higher efficiency. How-ever, this did not happen until several effec-tive approaches were developed in the early2000s, and the importance of morphologycontrol was soon realized to be a key com-ponent in OPV, which is far beyond what weknow from PPV system. Although P3HT isnot the highest efficiency OPV polymer anymore, it provides an excellent model systems

and the morphology study techniques startedon P3HT:PCBM system have long lastingimpacts.

Thermal annealing approach was shownby Friend et al. in early 2000 in OPV usingP3HT as donor and a small moleculeacceptor, but the efficiency was low (peakEQE 11%).[166]Camaioni et al. observed thata very mild annealing at 55 C can improvethe P3HT:fulleropyrrolidine solar cell from0.1 to 0.6% PCE.[167]Padinger, Sariciftci et al.further improved approach by introducingpost (electrode deposition) annealing (and

also anneal under bias) in a P3HT:PC61BMsystem.[168] The 3.5% efficiency achieved is beyond MDMO-PPV system and thus has high impact. Imperial College, [169]UCLA,[170] UCSB,[87] Wake Forrest[171] groups also conductedinfluential work on the thermal annealing approach. It isrealized that although P3HT itself has excellent absorptionup to 650 nm, the long wavelength section (correlated to stacking of polymer chains) is severely suppressed with the pre-sent of acceptor PC61BM. The thermal annealing approach canpartially recover the polymer ordering and absorption. Anotherapproach solvent annealing was then reported by Li et al.[35]In this approach, the slower solvent evaporation rate allowspolymer RR-P3HT to self-organize and restore its high degreeof order even in the polymer:PC61BM blend.[36]The enhanced

absorption and transport enabled the high efficiency OPV.In addition to thermal and solvent annealings, a mixture-

solvent approach represents another promising method tomodify solar cell morphology and improve light-harvestingefficiency. Zhang et al. found a significant enhancement inphotocurrent density in polyfluorene copolymer/fullereneblends when introducing a small amount of chlorobenzeneinto chloroform solvent.[172]Time-resolved spectroscopy on thepicosecond time scale shows that charge mobility was influ-enced by the mixing solvents. Bazan et al. reported that whenworking with an amorphous low band-gap polymer PCPDTBT,incorporating a few volume percent of alkanedithiols into the

from 0.9% in Toluene case to 2.5% in CB case, with excellentexternal quantum efficiency above 50% achieved. The highersolubility of PC61BM in CB was proposed to be responsible forbetter morphology and thus device efficiency. This is a goodexample of solvent selection in the controlling of polymer mor-phology. Figure 8c,8d show the SEM cross-section views of theMDMO-PPV:PC61BM system casted from chlorobenzene andtoluene, respectively. A 2040 nm thick skin layer (identifiedas polymer nanospheres) was observed in the toluene-castedfilm, which covers PC61BM nanocrystallites. However, formost chlorobenzene-casted films, the polymer nanospheres arehomogeneously distributed. The morphology difference in ver-tical direction may be linked to exciton dissociation and charge

transport.Later studies further showed that the solubility of the

fullerene could affect the solvent selection strongly. Largefullerenes tend to be less soluble; different solvents have beenused for optimal processing conditions. For example, MDMO-PPV:PCBM solar cells were typically spun from CB solutionfor easier smooth film formation, and for higher efficiency,PC71BM, was introduced for MDMO-PPV system. A record3.5% efficiency OPV was reported 1,2-diclorobenzene (DCB) assolvent.[147,163,164] CB and DCB are now the most popular sol-vents used for OPV research. It needs to be pointed out thatthese solvents are not good solvents for manufacturing, and

Figure 8. (a) AFM images of the MDMO-PPV:PCBM films cast from toluene and (b) CB. (c) SEMcross-section views of the MDMO-PPV:PC61BM films casted from toluene and (d) from CB.(e) AFM phase image of high crystalline P3HT:PC61BM films achieved using solvent annealingand (f) additive approaches. (g) BF-TEM images of PTB-7:PC61BM film casted using CB asmajor solvent with and (h) without DIO as additive. (i) BF-TEM and (j) defocussed phasecontrast TEM images of multiply polymer (blend) layers. (a,b,c,d) Reproduced with permis-sion from.[162]Copyright 2001 American Institute of Physics. (e,f ) Reproduced with permissionfrom.[36]Copyright 2007 Wiley VCH. (g,h) Reproduced with permission from.[107a]Copyright2010 Wiley VCH. (i,j) Reproduced with permission from.[173]Copyright 2012 Wiley VCH. (k)Reproduced with permission from.[175] Copyright 2009 American Chemical Society. (l) Repro-duced with permission from.[176]Copyright 2009 American Chemical Society.

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as a nano-scale 3-D imaging tool recently.[174]Andersson et al.showed 3-D nanostructure of a APFO-3:PC61BM solar cell usingelectron tomography (Figure 8k).[175] The electron scatteringfrom the film of pure phase of PC61BM and polymer is observedto be considerably different (by a ratio of 3). This allows the use

of the differing content of PC61BM to show 3-D nanostructure.In the reconstruction of the 3-D image of the blend materials inFigure 8k, the highlighted (scattering) domains are PC61BM richvolumes with APFO-3 rich volumes as surrounding materials.Loos et al. studied 3-D morphology in MEH-PPV:PC61BM andP3HT:PC61BM systems.[176] The contrast between crystallizedpolymer nanostructures with PCBM (Figure 8l) enables the esti-mation of P3HT crystallinity (60%). The volume percentageof crystalline P3HT nanorods varies in the vertical direction,indicating a more delicate 3-D morphology.

A further improvement of TEM contrast is enabled by usingenergy-filtered TEM (EF-TEM).[177]The contrast is based on thelocal material electronic signature via electron energy loss spec-trum and the EF-TEM images were collected using electrons

within a specified energy loss window. Indeed, polymers andfullerenes turn to have different plasmon energy loss windows.This provides an effective way to get detailed morphology in thesubtle BHJ films. Figure9a shows the electron energy loss spec-troscopy (EELS) result of pristine P3HT and PC61BM films.[178]Based on EELS result, Figure 9b, 9c and 9d show the EF-TEMimages filtered at 0V (conventional BF-TEM), 19 eV (4 eV) and30 eV (4 eV) of P3HT:PC61BM film. Bright regions in Figure 9ccorrespond to P3HT-rich region while the bright regions inFigure 9d are PC61BM-rich. Fibular features P3HT domain areclearly visible in all figures, and 9c and 9d are basically inverted.Very recently, Yang et al showed EF-TEM in a more amorphous

PCPDTBT/PC71BM polymer blend solution can double the PCEfrom 2.8% to 5.5%.[37]The doubled performance was a resultof the enhanced interactions between the polymer chains and/or between the polymer and fullerene phases upon alkanedith-iols addition, which was evidenced by the absorption data. Theadditive approach is also useful for P3HT system.

Characterization of the polymer morphology involvesmultiple technologies. Microscopic techniques provide a directview of polymer morphology. Atomic force microscopy (AFM)in tapping mode is suitable for soft PSC films and can providehigh-resolution surface topography and surface D-A distribu-tion on the nanoscale. Figure 8e and 8f show the phase imageof high-crystalline P3HT:PC61BM films achieved using solventannealing and additive approaches, respectively.[36]The polymernanofibrillar structure is consistent with that in pure P3HTfilm. P3HT nanofibrillar width is 2030 nm, consistent withthe morphology model.

Another powerful imaging technique is transmission elec-tron microscopy (TEM). A P3HT:PCBM film was studied usingbright field TEM (BF-TEM) technique shown in Figure 8g.[173]

The specific density difference of P3HT and PC61BM (1.1 vs.1.5 mg/cm3) enables the mapping of polymer and fullerenerich regions, providing information on the dimensions ofthe P3HT nanostructure. Loos et al. thus showed in the ther-mally annealed P3HT:PC61BM film, fibril structured P3HTis clearly seen, and PC61BM is showed as darker region inthe TEM image. One of the major breakthroughs in the fieldbeyond P3HT is the BDT-TT based polymer. Yu and Li et al.showed 78% PCE can be achieved using the new polymer.Figure 8g and 8h shows the BF-TEM images of PTB-7:PC61BMfilm casted using CB as major solvent with and without DIO asadditive.[107]Large domains (about 100200 nm in diameter) inthe blend film were observed in CB case, which is expected to

diminish exciton migration to the donor/acceptor interface andthus is not favorable for charge separation. The morphology ofblend film prepared from CB/DIO is much more uniform withno large phase separation. However, fibril polymer structuresare clearly seen, showing good miscibility between PTB-7 andPC71BM and the formation of interpenetrating networks. TheDIO additive indeed leads to much improved performance withthe fine-tuned morphology.

Cross-section TEM provides another critical piece of mor-phological information. Heeger et al. showed cross-sectionTEM images of a P3HT:PC61BM solar cell, where bicontinuousinterpenetrating polymer and fullerene domains were eluci-dated.[172]In Figure 8i and 8j, UCSB group further showed thedifference in conventional BF-TEM and defocussed phase con-

trast TEM images of multiply polymer (blend) layers, includingP3HT:PCBM, PCDTBT and PCDTBT:PCBM bulk heterojunc-tion.[173]The combination of surface topological and cross sec-tion imaging tools provide strong evidence for the morphologyof quasi-optimized polymer:PC61BM solar cells. One has tokeep in mind that the valuable polymer/PC61BM morphologyinformation needs very careful defocused phase contrast TEM,which otherwise may easily have artifacts.

Originally developed in life science, electron tomographyis used to reconstruct 3-D objects from a series of 2-D imagesthrough sequential tilting of the sample about a single axis (tiltseries). It has also been applied to polymer blends using BF-TEM

Figure 9. (a) EELS results of pristine P3HT and PC61BM films. (b) EF-TEMimages filtered at 0V (conventional BF-TEM), (c) 19 eV (4 eV) and(d) 30 eV (4 eV) of P3HT:PC61BM film. Reproduced with permissionfrom.[178]Copyright 2011 American Chemical Society.

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4.3. Molecular Level Structural Order in BHJ Thin Films

Grazing-incidence X-ray diffraction (GIXRD) is able to detect thedetails of crystallinity structure in thin films. Figure10a showsthe GIXRD data of as-cast and thermally annealed P3HT:PCBMfilm by Erb et al., where it can clearly see the enhancementof polymer crytallinity after thermal annealing.[184] However,the weak polymer crystallinity requires a synchrotron beam-line with high X-ray photon flux and collimation for accuratestructure information.[185] Figure 10b shows the synchrotron

GIWAXS results of conventional non-annealed and thermalannealed P3HT (95% RR):PC61BM. A clear improvement incrystallinity is clearly seen. Solvent annealed P3HT:PC61BMfilm, which showed much higher vibronic stacking peaksin absorption, was also examined using Synchrotron X-raysource (Figure 10c).[36] The prominent (n00) diffraction peaksindicates high crystallinity with dominantly edge-on chain ori-entation. The high resolution GIWAXS signal also enabled theelucidation of polymer inter-chain distance spacing reductionin solvent annealing approach (from 16.9 in fast grown filmto 16.3 in solvent annealed film), which can be translated intoimproved carrier transport in OPV device.

PDTP-DFBT:PC61BM blend film, where thecontrast of polymer and fullerene is not ashigh as in the P3HT:PCBM system.[46]

4.2. Vertical Phase Separation in Donor/

Acceptor Blend Fims

Although the polymer blend film is castedfrom uniformly distributed donor/acceptorsolution, vertical phase separation hasalso been reported in a variety of semicon-ducting polymer blend systems. Bjrstrmet al. observed a multilayer formation afterspin-coating APFO-3 blended with PC61BMin chloroform by dynamic secondary ionmass spectroscopy (SIMS).[179] The verticalstructure exhibited a four-fold multilayermorphology with APFO-3 enriched at thetop surface, followed by a PC61BM-enrichedlayer underneath, then a APFO-3-enrichedlayer in the middle, and a PC61BM-enriched(APFO-3-depleted) adjacent to the siliconsubstrate.

Campoy-Quiles et al. used variable-angle spectroscopic ellipsometry (VASE)to model the vertical composition profileof P3HT:PC61BM thin films casted fromvarious preparation methods, and reporteda common vertically-and laterally-phaseseparated morphology, independent of thepreparation techniques. A concentrationgradient varying from PC61BM-rich near the

PEDOT:PSS side to P3HT-rich adjacent to thefree (air) surface was consistently observed.Another way is using XPS measurement

on the top and bottom surfaces of the activelayer to determine the polymer/fullerene composition. Yanget al. used the floating off method in water to peel off the filmand put it on a TEM grid. The results of peak area ratios of theS(2p) (signature of polymer) and C(1s) (total content of P3HTand PC61BM) peaks for the top and bottom surfaces were com-pared. It is found that fast coated P3HT:PC61BM has a homo-geneous distribution of P3HT in the vertical direction. How-ever when add OT additive, the top surface is P3HT rich. Yanget al. further improved the accuracy of this approach by usingF-PC61BM to replace PC61BM, and using F-atom signature to

trace the fullerene distribution.[180] This further strengthenedthe conclusion in P3HT:PC61BM device.

In addition to the P3HT rich region on the top surface, a near-edge X-ray absorption fine structure spectroscopy (NEXAFS)study by DeLongchamp et al. showed that P3HT concentra-tion at the buried interface depends strongly on the substratesurface energy.[181]Similar results were also obtained in recentneutron reflectometry (NR) studies.[182]Chu et al. applied con-focal optical microscopy combined with a fluorescence moduleto study the exciton lifetime in thick P3HT:PC61BM films(2 um), which provides certain 3-D morphology informationfor OPV.[183]

Figure 10. (a) GIXRD data of as-cast and thermally annealed P3HT:PCBM. (b) GIWAXS resultsof conventional non-annealed and thermal annealed P3HT (95% RR):PC 61BM. (c) GIWAXSresults of solvent annealed P3HT:PCBM film. (d) GIXRD results of BDT-TPD copolymers withdifferent side chains. (e) GIWAXS images of pure PTB-7 film, and PTB-7:PC61BM films castedfrom CB and CB + DIO, respectively. (a) Reproduced with permission from.[184]Copyright 2005Wiley VCH. (b,c) Reproduced with permission from.[36]Copyright 2007 Wiley VCH. (d) Repro-duced with permission from.[111] Copyright 2010 American Chemical Society. (e) Reproduced

with permission from.[186b]Copyright 2011 American Chemical Society.

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5. Interface Engineering & Inverted DeviceStructure

After the electrons and holes are separated at the BHJ interface,the collecting electrode should effectively drain the charges andmove them to the external circuit. The organic/metal contacts

of an OPV device play critical role for determining the deviceperformance. An electrical contact of barrier height of fewtens of meV can result in significant charge accumulation andthus inferior photovoltaic performance. Therefore, it is nec-essary to establish ohmic contacts on both sides of the anodeand cathode contacts for efficient charge extraction with highselectivity. In this section, we will discuss the interface engi-neering of the anode and cathode contacts; and the formationof inverted device architecture.

In the typical OPV structure, or the rigid band metal-semiconductor-metal model, the difference in work functionbetween the anode and cathode provides the driving force forthe charge transport and extraction process. (See Sction 2 ofthis manuscript.) Tremendous efforts have been devoted to

modify interfaces between organic materials and metals forimproving efficiency, device stability, and simple fabrication.This is often done by inserting of organic (such as conductingpolymer or poly-electrolyte) or inorganic (such as metal oxides)layers between the metal and organic materails. In addition,the insertion of interfacial layers offers an effective approach tofinely tune the optical properties in the multi-layer devices, soas to position the photoactive layer to maximize the optical fieldstrength or to change the exciton distribution to be favorablefor charge extraction.[191]

In this section, we will introduce the principles that domi-nate the electrical processes at interfaces and summarize theprogress in interface engineering. The interface engineering

has led to the invention of inverted device architecture, whichhas shown promising engineering advantages in industrialscale fabrication.

5.1. Energy Level Alignment at Interface

Polymer-fullerene blend films are generally formed onto a sub-strate surface from the solution phase, and it inevitably causescontamination of hydrocarbons and/or native oxides. The con-taminated films physically and electronically decouple thepolymer film from the continuum of the electronic states inbulk metal layers. Consequently, vacuum level alignment, i.e.SchottkyMott limit, holds at the polymer/electrode interface and

the barrier for electron injection equals the difference betweenwork function of the substrate and the LUMO of the polymer.Vacuum level shift occurs after the polymer film is deposited onthe conductive substrate, which is explained using the integercharge transfer (ICT) model as shown in Figure11.[192]

Although the contamination layer decouples the polymerlayer from the substrate, charge transfer can still occur via tun-neling as long as the contamination layer is sufficiently thin.Since the conjugated polymers are electronically soft mate-rials, adding or withdrawing charges induces substantial elec-tronic and geometric relaxation effects, leading to self-localizedpolaronic (single charge) or bipolaronic (double charge) states.

Wei et al. further studied the P3HT:PC61BM system withand without annealing using a combination of GIWAXS andGISAXS, which provides both the lattice spacing parametersand statistically averaged morphology information such asgrain sizes.[186a] They showed there is a correlation betweendevice performance with the sizes of PC61BM cluster (from

GISAXS) and P3HT crystallites (from GIWAXS). It is pro-posed that PC61BM cluster radius of gyration (Rg) need tobe larger than 20 nm, and P3HT crystallites greater than16 nm give high efficiency in thermally annealedP3HT:PC61BM devices.

The GIXRD technique is also applied to new high-efficiencylow bandgap copolymer systems. In some BDT-containingpolymer:PC61BM systems, face-on orientation is clearly seen.Figure 10d shows the GIXRD results of BDT-TPD copolymerswith different side chains.[111] Large planar BDT unit prefersface-on orientation in film formation. This indicates differentpolymers need different orientations for high performance,which requires customization in film processing. In Figure 10e,Chen et al. shows the GIWAXS images of pure PTB-7 film, and

PTB-7:PC61BM films casted from CB and CB + DIO, respec-tively. They showed that the polymer packing is dominantlyface-on in both pristine polymer and polymer blend. Usingvarious scattering techniques, including GIWAXS, resonancesoft x-ray scattering (RSoXS), specular X-ray reflectivity (XRR)etc., the authors proposed a multi-length scale morphologymodel of a PTB-7:PC61BM solar cell. This includes (1) PTB7crysatllites of a few nm surrounded by polymer and fullereneintermixture; (2) PTB-7 crystallite of tens of nm, amorphouspolymer-fullerene surrounded by polymer rich regions of200 nm.[186b]

X-ray diffraction techniques have also been used to provideinsight of polymer-fullenerne interactions inside the film. One

example is from McGehee et al. They showed that poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene (PBTTT) andPC71BM forms a bimolecular crystal, and the PC71BM mol-ecules intercalate between the PBTTT side chains.[187] Theconformation of the polymer is significantly disrupted by theincorporation of the fullerene molecules, which introducetwists and bends along the polymer backbone and fullerenechannels. The existence of mixed phase is correlated to reducedcell efficiency due to poor carrier transport.[188]

Recently there have been a few interesting in situ X-ray inves-tigation of polymer blend film formation process. Schmidt-Hansberg et al.[189]reported an in situ real-time GIWAX studyon the composition dependence during the drying of doctor-bladed P3HT:PC61BM film. They found that during the blend

film formation, when a large amount of PC61BM is present(P3HT:PCBM = 1:2), it impedes the P3HT crysatlization, andan interesting diffraction signal with 12.6A spacing is observed.This is attributed to a new disordered phase of intimate mixedP3HT:PC61BM. Amassian et al. conducted time-resolvedGISAXS and GIWAXS together with in situ spectral reflectom-etery to study the P3HT:PC61BM thin film formation during amuch faster spin-coating process. Both polymer crystallizationand phase separation information can be derived.[190]These in-situ efforts are expected to be very valuable for the developmentof OPV technology, as they can be used to closely related to thereal-world device/module fabrication process.

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the ORG/SUBin this case is also independentof SUB. Thus, when the work functionof the conductive substrates spans a suffi-ciently large range, there is a clear depend-ence between SUBand the work function ofthe organic-on-substrate interface ORG/SUB

with sharp transitions between the vacuumalignment (Shottky-Mott limit, EICT EICT+: Fermi-level pinning to a positiveinteger charge-transfer state, and (c) SUB


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in inverted device were observed and the high PCE was ben-efitted from the hydrophobic surface and the suitable electronicenergy levels of TIPD. Some polar materials with small thick-nesses or even mono-layers are able to induce a surface dipoleat the metal/polymer interfaces and shift the work functionof metals to a desirable level. Some of those have been found

to outperform the traditionally used materials. For example,Huang, Wu, Cao, Jen and Bazan et al. demonstrated that avery thin layer of conjugated polyelectrolyte (such as PFN) canenhance the electron extraction significantly in OPV devicescompared to the those without it. [42,44,201,202]Recently, grapheneand graphene oxide have also been applied in BHJ solar cells asboth hole and electron transporting layers due to their properwork function, high optical transmittance, and high electricalconductivity. [203208] It is encouraging that reasonable perfor-mance has been achieved and future work should be able tofurther enhance the efficiency.

5.3. Inverted Device Configuration

In principle, the electrical polarity of polymer photovoltaic cellcan be determined by the interfacial layer of the electrode. Forthe conventional device, a p-type buffer layer is deposited onthe transparent anode (glass/ITO, high work function), fol-lowed by the active layer, then an n-type buffer layer, and finallythe cathode (metal electrode). For the inverted device, an n-typebeffer layer is deposited on the transparent cathode (glass/ITO), followed by the active layer, then a p-type buffer layer, andfinally the anode (metal electrode). Yang and Li et al. in 2006 [209]first demonstrated the concept of using a low workfunctioninterficial layer (Cs2CO3) to lower ITO work function, andtransition metal oxides (e.g. V2O5, MoO3, WO3)[195a] as a holebuffer in OPV device to form inverted polymer solar cell. White

et al. at NREL shortly after showed ZnO can be used to forminverted polymer solar cells.[210] ITO/ZnO/active layer/MoO3/Metal is now the standard inverted structure. Nowadays,the inverted structure has become equally important as theconventional structure owing to the flexible structural design,materials selection, and a powerful approach to achieve highphotovoltaic performance, including efficiency, stability, etc. Forexample, So et al. showed that a modification on ZnO surfaceby PVP can enhance the electron extraction efficiency greatly.It was interesting to find that the use of PVP as an organiccapping molecule and polymeric matrix for ZnO can produceelectron-transporting nanocomposite films with excellent film-forming characteristics.[114] When a high performance ger-mole-based polymer was used as the active layer, photovoltaic

efficiency over 8% was reached (certified by Newport Inc. PCE= 7.4%). More recently, Wu and Cao et al. demonstrated thatwhen using a thin layer of polyelectrolyte (PFN) as a cathodebuffer, the PCE was dramatically enhanced to 9.2% based onPTB7:PC71BM system (VOC= 0.754 V, JSC= 17.5 mA/cm2, FF= 70.0%) with EQE approaching 80%.[44]The device structureis shown in Figure 12c. The work function of the ITO wasreduced from 4.7 eV to 4.1 eV, and the authors suggested thatthe orientation of PFN on ITO with a permanent dipole was themajor cause. The simultaneous enhancements of VOC, JSC, andFF can be attributed to this interfacial dipole. It is worth notingthat the authors predicted that a further increase of 10% in JSC

exposure. The good news is that even a 5.3 eV workfunctionis enough for a majority of new polymers. This makes it effec-

tive in OPV application. Doping MoO3with aluminum has alsobeen shown to be effective in finely tuning the work function soas to fit various purposes.[198c]

In contrast to anode contact layers, which have high workfunctions and relatively good stability against ambient expo-sure, the cathode contact buffer consisting of low work func-tion metals is rather sensitive to oxygen and moisture and thusshould be avoided. Alternatively, n-type metal-oxides, such asZnO and TiOx, have been widely adopted due to their superiorstability in terms of both film morphology and electronic prop-erties. Heeger et al. using TiOx as the electron collection layerand the cathode buffer improved photovoltaic efficiency to 6%PCE for PCDTBT based devices.[94]Later, the same group dem-onstrated sol-gel processed ZnO as the electron collection layer

for high performance OPV device based on the same polymer.PCEs approaching 7% were obtained due to the efficient chargecollection of the ZnO layer.[199a]Yang and Li introduced anataseTiOx nano-particles as an electron buffer material, and foundthat mixing TiOx with Cs2CO3leads to Cs-doping of TiOx, andthis results in a more reliable electron buffer material for OPVand OLED devices.[200a]Yang and Li et al. further did a thoroughcomparison of different n-type metal oxide based interfacelayers and found that ZnO is the best due to better performance,repeatability, and stability of the devices (Figure12a,12b).[200b]Recently, titanium chelate TIPD as cathode buffer material wasreported by Li et al.[200c]Significant JSCand PCE enhancement

Figure 12. (a) Device structure of the inverted cells using different elec-tron transporting materials. (b) Device efficiency and stability based ondifferent electron transporting materials. (c) Schematic of the inverted-type polymer photovoltaic cells, in which the photoactive layer is sand-wiched between a PFN-modified ITO cathode and an Al,Ag-based topanode and chemical structures of PTB7 and PFN. (a, b) Reproduced withpermission from.[200]Copyright 2012 Wiley VCH. (c) Reproduced with per-mission from.[44]Copyright 2012 Nature Publishing Group.

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higher than that of each sub-cell. In 2007, Kim et al. used thesolution processed TiO2(n-type)/PEDOT:PSS(p-type) intercon-necting layer to bridge two higher performance single cells torealize a tandem structure.[40] The front cell active layer is alow bandgap PCPDTBT and PC61BM blend, and the rear cellis made of P3HT and PC71BM. With the front and rear cell

of 3.0% and 4.7% efficiency, respectively, a 6.5% efficiency oftandem solar cell was achieved. The low band gap polyme

a decade of organic polymeric photovoltaic research

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